Now fast-forwarding to this vehicle, I can fully see how if Murphy’s Law applied to too many components in this brake-by-wire system, you might not have much, if any, braking ability. Looking under the hood at the Prius’ braking system, I saw a pump on the Brake Actuator (ABS modulator assembly in my mind) located just behind and under the hybrid inverter/converter assembly. The brake actuator assembly contains the usual solenoids for ABS/TCS duties plus the typical electric pump motor, along with a pressure accumulator and pressure sensor. There is also something called a linear solenoid valve that does as the name implies – applies varying amounts of braking force instead of full on or off like a system completely controlled with solenoids so that the apply of the brakes via the driver’s foot or even TCS/VSC can be accomplished with a bit of progressive finesse. (No bouncing pedal.)
Hey, I have a bouncing pedal on this car! Another critical part I discovered on this car was the back-up power supply (22 12-volt capacitors in parallel in a black box) next to the 12-volt battery in the right rear fender/interior section of the car. That makes sense. If you have a brake-by-wire system running on 12-volts what happens if the power goes down? You will need to have some backup power much like airbags do with their energy storage modules.
To give the driver seamless transition between brake-by-wire, regenerative braking and back-up hydraulic braking, there is a master cylinder stroke simulator. No matter what the case is for braking, the driver gets the same feel at the pedal. The last and most critical part of this challenging system I noticed was the master cylinder stroke cut valve. It’s important role in this system is to cut (or should I say redirect) the flow of master cylinder applied hydraulic pressure from the stroke simulator into the actual brake lines going to the front calipers and rear wheel cylinders. The Skid Control Module controls this master cylinder stroke cut valve. Now it all starts to make sense, but what about the other DTCs for things like relay problems and so forth?
Let’s Get Serious About Diagnostics
Enough theory; my head is hurting! I called a Toyota factory hotline acquaintance and posed the problem to him. I also posted the problem on iATN (International Automotive Technicians Network) and sent emails out to a couple of other trainers who specialize in hybrid vehicles. The response from the group made sense: multiple DTCs in the same system are often the result of wiring issues such as power and ground problems.
Let’s look at that suspicious ground right off the bat. It didn’t look all that bad when it came off, but I gave it a good cleaning and tightening. No change. Next was checking the power feeds to the relays, the Skid Control Module (main brain) and brake actuator assembly. There was good continuity to the grounds and 12-plus volts to each component. “But what about a load induced voltage drop,” I thought to myself.
I grabbed a load simulator box and wired 1 ohm in each high current circuit powered up. Every circuit seemed good with a tenth or less of a volt of voltage drop except for the circuit supplying the brake actuator module. Getting to that module, by the way, involved powering down the high voltage system by removing the hybrid battery pack service plug (sans 1000 volt class 0 gloves) and removing the inverter cover (after the wipers and cowling are removed) so that I could measure and verify the absence of high voltage (those big capacitors should discharge).
My arm is going to be lying across that inverter (cover removed) just to remove and probe the brake actuator module’s connector and I don’t want any nasty surprises.